The overall goals of the proposed research are to define the role of expiratory braking mechanisms in the neonatal strategy to maintain an elevated end-expiratory lung volume above the mechanically determined relaxation volume of the respiratory system, and to develop greater basic understanding of interactions between chemical and mechanoreceptor control of expiratory braking mechanisms and respiratory timing modulation in healthy full-term newborn infants. To attain these goals five specific aims will be pursued. First, the role of spontaneous expiratory braking in the eupneic breathing strategy will be determined in awake and sleeping infants using both neural and mechanical noninvasive methodologies to differentiate between braking caused by post-inspiratory inspiratory muscle activity and dynamic narrowing of the upper airway. Second, using small resistive loads during single expirations, the hypothesis that the infant is able to make reflex adjustments in braking to compensate for sudden changes in expiratory resistance will be tested. Third, the hypothesis that infants reduce braking in response to a hypercapnic increase in ventilatory drive will be tested. Fourth, using brief pulse inflations at varied times during single expirations, the dynamic characteristics of the expiratory-phase inflation reflex will be determined under normo and hypercapnic conditions. Fifth, by comparing the effects of CO2 inhalation to induced single-breath augmented inspirations on the inspiratory-phase inflation reflex, three experiments will be performed to: (i) test the hypothesis that infants modulate inspiratory time in the volume range above the mean eupneic tidal volume, (ii) test the hypothesis that the increase in inspiratory "off-switch" threshold is greater than the increase in the rate of rise of central inspiratory activity during hypercapnia in the infant, and (iii) define the effect of hypercapnia on the shape and position of the declining inspiratory "off-switch" threshold curve. The successful completion of the proposed research will provide new means for refining and extending existing hypotheses regarding the neuromuscular control of normal infant breathing. This information will be of importance to the study of unstable respiratory patterns and apnea in premature infants and in patients adapting to specific effects of neurological or respiratory illness.